1. Field of the Invention
The present invention relates to a method and apparatus for detecting a frame alignment word in a data stream.
2. Description of Related Art
The invention finds utility in digital multiplex systems, and is generally applicable to digital communication systems where the detection of specific, recurring binary sequences is required under condition of medium to high binary error ratios.
In digital multiplex systems several independent tributary data streams are combined to a higher rate aggregate bit sequence. A specific set of characters, known as the Frame Alignment Word (FAW), is then inserted at regular intervals into the bit sequence of the aggregate signal prior to transmission. The FAW together with the subsequent bit sequence up to the start of the next FAW constitutes a digital frame.
At the demultiplexer, the received binary data is initially examined on a bit-by-bit basis until a FAW has been correctly detected. This process is known as Frame Search. When the frame search is complete a new digital frame is constructed which is a replica of the originally transmitted digital frame, the transmitted and received digital frames are then said to be in alignment. When the demultiplexer frame is in alignment the inverse of the multiplex procedures can be applied to reconstitute the original data streams.
In order to maintain correct operation of the demultiplexer it is necessary to continuously check the occurrence of a FAW in the expected position in the digital sequence to confirm that frame alignment is being maintained. When the check procedure indicates loss of alignment a new frame search is initiated.
In practical digital transmission systems binary errors cause the corruption of the FAWs resulting in failure to recognize the FAW during frame alignment procedures and spurious detection of misalignment when the digital frame is already aligned. The probability of a corrupted FAW is dependent on the binary error ratio and the number of bits which constitute the FAW. The larger the error ratio and the number of bits in the FAW the greater the probability of corruption.
In many applications the FAW is constructed to give a sufficiently long binary sequence so that the probability of its pattern being simulated by a combination of data bits within the digital frame is negligibly small. Therefore a demultiplexer can readily identify a FAW within the received signal, using a template matching technique. By this technique the incoming data stream is compared, on a bit-by-bit basis, against a template pattern of the FAW; if there is a match between the incoming stream and the template pattern, then a FAW is declared as recognized.
If the detection of the FAW is based on an exact match between the incoming data stream and the FAW template pattern, it cannot recognize valid FAWs if they have been corrupted by digital errors. Under such conditions, the demultiplexer cannot achieve rapid frame alignment or, in the case of higher error rates, may be subject to frequent spurious realignment thus greatly increasing the impairment of the received digital signal.